[Upgrade] Mention entrypoint import

UPGRADE-2.0.md

@@ -24,20 +24,29 @@
        bin/console doctrine:migrations:migrate --no-interaction
    ```
 
+1. As the assets have been reorganized, you need to update the way you import them in your application.
+   Add the following line to your `assets/admin/entrypoint.js` file:
+
+    ```js
+        import '../../vendor/sylius/refund-plugin/assets/entrypoint';
+    ```
+
 1. The structures of the directories have been updated to follow the current Symfony recommendations:
    - `@SyliusRefundPlugin/Resources/assets` -> `@SyliusRefundPlugin/assets`
    - `@SyliusRefundPlugin/Resources/config` -> `@SyliusRefundPlugin/config`
    - `@SyliusRefundPlugin/Resources/translations` -> `@SyliusRefundPlugin/translations`
    - `@SyliusRefundPlugin/Resources/views` -> `@SyliusRefundPlugin/templates`
 
    You need to adjust the import of configuration file in your end application:
+
    ```diff
    imports:
    -   - { resource: "@SyliusRefundPlugin/Resources/config/config.yml" }
    +   - { resource: '@SyliusRefundPlugin/config/config.yaml' }
    ```
 
    And the routes configuration paths:
+
    ```diff
        sylius_refund:
    -       resource: "@SyliusRefundPlugin/Resources/config/routing.yml"